Add meta.c which implements "meta" mode for make.
In this mode, a .meta file is created for each target, capturing
the expanded commands used, any command output, and if filemon(9)
is available, a record of system calls which are of interest.
Not enabled unless USE_META=yes is set when building make.
Also, if FILEMON_H exists, meta.c will be compiled to use filemon(9).